Electrostatographic printing operates according to the principles and embodiments of non-impact printing as described, eg, in "Principles of Non-Impact Printing" by Jerome L Johnson (1986)--Palatino Press--Irvine Calif., 92715 USA).
Electrostatographic printing includes electrographic printing in which an electrostatic charge is deposited image-wise on a dielectric recording member as well as electrophotographic printing in which an overall electrostatically charged photoconductive dielectric recording member is image-wise exposed to conductivity increasing radiation producing thereby a "direct" or "reversal" toner-developable charge pattern on said recording member. "Direct" development is a positive-positive development, and is particularly useful for reproducing pictures and text. "Reversal" development is of interest in or when from a negative original a positive reproduction has to be made or vice-versa, or when the exposure derives from an image in digital electrical signal form, wherein the electrical signals modulate a laser beam or the light output of light-emitting diodes (LEDs). It is advantageous with respect to a reduced load of the electric signal modulated light source (laser or LEDs) to record graphic information (eg printed text) in such a way that the light information corresponds with the graphic characters so that by "reversal" development in the exposed area of a photoconductive recording layer, toner can be deposited to produce a positive reproduction of the electronically stored original. In high speed electrostatographic printing the exposure derives practically always from electronically stored, ie computer stored information.
As used herein, the term "electrostatographic" also includes the direct image-wise application of electrostatic charges on an insulating support, for example by ionography.
In the electrophotographic art, an electrostatographic single-pass multiple station multi-colour printer is known, in which an image is formed on a photoconductive belt and is then transferred to a paper receiving sheet or web whereon the toner image is fixed, whereupon the web is usually cut into sheets containing the desired print frame.
In an alternative printer, toner images are transferred to an insulating belt from distinct image forming stations and are then transferred to the receiving sheet or web and fixed thereon. In U.S. Pat. No. 5160946 (Hwang assigned to Xerox Corporation) there is described an electrophotographic printing machine in which a plurality of image-forming units are arranged to superimpose toner images onto a motor-driven endless belt, from which the superimposed image is transferred to a paper sheet. Each image-forming unit includes a rotatable drum driven by a motor (see column 5, lines 22 to 27) in synchronism with the endless belt.
It is desirable to transfer a plurality of toner images in succession, that is in a single pass through the printer, directly onto the receiving web. In order to achieve this, accurate registration of the images with each other is required, ideally to an accuracy of about 40 .mu.m, or better. In order to achieve this registration accuracy, it is essential that there should be no slippage, ie synchronism, between the web and the image bearing surface. When, for example, a number of rotatable drums are driven by individual motors, it is found in practice to be difficult to obtain perfect synchronous movement between the drums and the receiving web--resulting in registration errors.